The 5-year survival of patients with advanced head and neck cancer with current therapeutic protocols is very low. Clinical benefit is sometimes observed when anti-angiogenic strategies are combined with standard chemotherapy regimens. However, even in such cases, the survival benefits are modest. The role of anti-angiogenic therapy in the sensitization of head and neck tumors to standard chemotherapy remains unclear. Improvements in the clinical outcome of patients with head and neck cancer are likely to come from a better understanding of mechanisms underlying the overall impact of endothelial cells in tumor pathobiology. Physiological stem cells (e.g. neural stem cells) reside within vascular niches and depend on endothelial cell-derived signaling for their survival. Notably, vascular endothelial growth factor (VEGF) was shown to enhance the survival of hematopoietic stem cells. Tumors, including those of the head and neck, are also populated with stem cells. We have recently shown that neovascular endothelial cells secrete VEGF and that perivascular cells co-express VEGFR1 and stem cell markers (i.e. CD44, ALDH) in human head and neck tumors. The role of stem cells in tumorigenesis is still unclear, but an emerging paradigm proposes that cancer stem cells drive tumor recurrence because they are resistant to conventional chemotherapy. Here, we hypothesize that endothelial cell-secreted VEGF contributes to the stemness and survival of head and neck cancer stem cells, and enhance the resistance of head and neck tumors to chemotherapy with Cisplatin. To test this hypothesis, we propose the following Specific Aims: 1) to evaluate the effect of endothelial cell-initiated VEGF/VEGFR1 signaling on HNCSC stemness and survival;and 2) to study the effect of endothelial cell-initiated VEGF/VEGFR1 signaling on HNCSC survival in xenograft head and neck tumors, and in resistance to chemotherapy with Cisplatin. Head and neck cancer kills 40,000 Americans every year. The majority of these deaths have been associated with the development of evasive resistance to therapy. Very recently, it has been suggested that cancer stem cells are intrinsically involved in resistance to chemotherapy and mediate tumor re-growth. The proposed research utilizes gene silencing strategies for the study of mechanisms regulating molecular crosstalk initiated by endothelial cells that contribute to survival and stemness of head and neck cancer stem cells. In addition, we will perform translational studies to understand the impact of endothelial cells on cancer stem cell-mediated resistance to chemotherapy. These studies will explore new drug targets for overcoming evasive resistance of head and neck tumors to conventional chemotherapy. Project Narrative: The relevance to public health of the proposed research lies in its potential to lead to improvements in the clinical outcomes of head and neck cancer patients. This is critical since the current benefits of therapy are modest for these patients, and their 5-year survival remains very low. The proposed research outlines strategies for obtaining such improvements by :1) studying the role of endothelial cells on the stemness and survival of head and neck cancer stem cells;and by 2) evaluating the role of crosstalk between endothelial cells and head &neck cancer stem cells on response to chemotherapy. The information derived from these studies might lead to the identification of drug targets for the sensitization of head and neck tumors to chemotherapy.